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A Strange Lonely Planet without a Star

Multicolor image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. The planet is extremely cold and faint, about 100 billion times fainter in optical light than the planet Venus. Most of its energy is emitted at infrared wavelengths. The image is 125 arcseconds on a side. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium.

An international team of astronomers has discovered an exotic young planet that is not orbiting a star. This free-floating planet is just 80 light-years away from Earth and has a mass only six times that of Jupiter. The planet formed a mere 12 million years ago—a newborn in terms of planetary lifetimes.

During the past decade, extrasolar planets have been discovered at an incredible pace, with about a thousand found by indirect methods such as wobbling or dimming of their host stars induced by the planet. However, only a handful of planets have been directly imaged, all of which are around young stars, less than 200 million years old. These planets are hard to study because they are right next to their bright host star. In order to understand the physical properties of these planets, astronomers compare them to cold and low mass brown dwarfs that are isolated thus much easier to study. The idea is that the smallest and coldest dwarf stars will have similar properties than those of the giant planets. However, efforts to link these two classes of objects together have so far been unsuccessful. These planets are truly unique objects.

Using the Pan-STARRS1 (PS1) wide-field survey telescope on Haleakala, Maui, an international team of astronomers identified an object with a faint and unique heat signature. Follow-up observations using IRTF, Gemini, UKIRT and CFHT on Mauna Kea show that it has properties similar to those of gas-giant planets found orbiting around young stars. This free-floating planet, dubbed PSO J318.5-22, is just 80 light-years away from Earth and has a mass only six times that of Jupiter. PSO J318.5-22 is one of the lowest-mass free-floating objects known, perhaps the very lowest. But its most unique aspect is its similar mass, color, and energy output to directly imaged planets.

The team did astrometric monitoring of PSO J318.5-22 obtaining 9 epoch over two years using WIRCam on CFHT. The resulting median astrometric precision per epoch is 4.0 milliarcseconds, a stunningly precise measurement for such a faint, cool object. These measurements are shown on the graph below. From this, the team was able to do a direct determination of the distance to PSO J318.5-22 and it's absolute J band magnitude, essential parameters for this type of study. Also, using the high precision astrometry provided by WIRCam, the team was able to conclude that PSO J318.5-22 belongs to a collection of young stars called the Beta Pictoris moving group that formed about 12 million years ago thus determining the age of PSO J318.5-22.

WIRCam stacked image of the newly discovered planet. This image is a stack of the individual images used to produce the plot on the left. Credits: CFHT, Dr. Trent Dupuy, Harvard-Smithsonian Center for Astrophysics.

Proper motion and parallax plot of PSO J318.5-22. The tiny dots are evenly spaced in time showing the predicted motion with the dates of the parallax maximum/minimum labeled. The measurements of the astrometric position are in blue, with error bars. For reference, the dashed purple line shows the size of a WIRCam pixel (~0.3 arcseconds). Credits: Dr. Trent Dupuy, Harvard-Smithsonian Center for Astrophysics.

The discovery paper of PSO J318.5-22 is being published by Astrophysical Journal Letters and is available from the arXiv. The key authors of the paper are the lead author, Dr. Michael Liu (Institute for Astronomy at the University of Hawaii at Manoa), Dr. Niall Deacon (Max Planck Institute for Astronomy, Germany), Dr. Katelyn Allers (Bucknell University), Dr. Trent Dupuy (Harvard-Smithsonian Center for Astrophysics), and Michael Kotson and Kimberly Aller (University of Hawaii at Manoa).